Signal Detection, Noise, and the Evolution of Communication

Birds living near airports do not show consistently higher levels of feather corticosterone

Noise represents a threat to human and wildlife health, triggering physiological and behavioral challenges to individuals living close to sources of extreme noise. Here, we considered airport environments as sources of potentially stressful stimuli for birds and tested if those living near airports are under higher physiological stress than birds living in quiet sites. We used measurements of CORT in feathers (CORTf) as a proxy of chronic stress. We evaluated 14 passerine and 1 non-passerine species, living near three Brazilian airports. We found that, across species, individuals with a better body condition had lower CORTf concentration. At the species level, we found that CORTf concentration was not consistently affected by airport noise. Comparing individuals living in quiet sites with those living near airports, we found that 2 species had higher and 2 had lower CORTf concentrations near airports, while 11 species presented no significant differences between sites. At the population level, model selection indicated that the direction and strength of these differences are weakly related to species' song frequency (peak frequency), as lower-frequency singers tended to present higher CORTf levels at airport-affected sites. In summary, we were unable to find a consistent response among species, probably due to species-specific differences in their response to anthropogenic disturbances. Instead, we found that species might be affected differently according to their singing spectral frequency and that individuals in good body condition show lower CORTf, suggesting that this measure is consistent with lower physiological stress.

The evolution of manipulative cheating

A social cheat is typically assumed to be an individual that does not perform a cooperative behaviour, or performs less of it, but can still exploit the cooperative behaviour of others. However, empirical data suggests that cheating can be more subtle, involving evolutionary arms races over the ability to both exploit and resist exploitation. These complications have not been captured by evolutionary theory, which lags behind empirical studies in this area. We bridge this gap with a mixture of game-theoretical models and individual-based simulations, examining what conditions favour more elaborate patterns of cheating. We found that as well as adjusting their own behaviour, individuals can be selected to manipulate the behaviour of others, which we term 'manipulative cheating'. Further, we found that manipulative cheating can lead to dynamic oscillations (arms races), between selfishness, manipulation, and suppression of manipulation. Our results can help explain both variation in the level of cheating, and genetic variation in the extent to which individuals can be exploited by cheats.

The exploitation of sexual signals by predators: a meta-analysis

Sexual signals are often central to reproduction, and their expression is thought to strike a balance between advertising to mates and avoiding detection by predatory eavesdroppers. Tests of the predicted predation costs have produced mixed results, however. Here we synthesized 187 effects from 78 experimental studies in a meta-analytic test of two questions; namely, whether predators, parasites and parasitoids express preferences for the sexual signals of prey, and whether sexual signals increase realized predation risk in the wild. We found that predators and parasitoids express strong and consistent preferences for signals in forced-choice contexts. We found a similarly strong overall increase in predation on sexual signallers in the wild, though here it was modality specific. Olfactory and acoustic signals increased the incidence of eavesdropping relative to visual signals, which experienced no greater risk than controls on average. Variation in outcome measures was universally high, suggesting that contexts in which sexual signalling may incur no cost, or even reduce the incidence of predation, are common. Our results reveal unexpected complexity in a central viability cost to sexual signalling, while also speaking to applied problems in invasion biology and pest management where signal exploitation holds promise for bio-inspired solutions.

Ecology and behavior predict an evolutionary trade-off between song complexity and elaborate plumages in antwrens (Aves, Thamnophilidae)

The environment can impose constraints on signal transmission properties such that signals should evolve in predictable directions (Sensory Drive Hypothesis). However, behavioral and ecological factors can limit investment in more than one sensory modality leading to a trade-off in use of different signals (Transfer Hypothesis). In birds, there is mixed evidence for both sensory drive and transfer hypothesis. Few studies have tested sensory drive while also evaluating the transfer hypothesis, limiting understanding of the relative roles of these processes in signal evolution. Here, we assessed both hypotheses using acoustic and visual signals in male and female antwrens (Thamnophilidae), a species-rich group that inhabits diverse environments and exhibits behaviors, such as mixed-species flocking, that could limit investment in different signal modalities. We uncovered significant effects of habitat (sensory drive) and mixed-species flocking behavior on both sensory modalities, and we revealed evolutionary trade-offs between song and plumage complexity, consistent with the transfer hypothesis. We also showed sex- and trait-specific responses in visual signals that suggest both natural and social selection play an important role in the evolution of sexual dimorphism. Altogether, these results support the idea that environmental (sensory drive) and behavioral pressures (social selection) shape signal evolution in antwrens.

Neurophysiology goes wild: from exploring sensory coding in sound proof rooms to natural environments

To perform adaptive behaviours, animals have to establish a representation of the physical "outside" world. How these representations are created by sensory systems is a central issue in sensory physiology. This review addresses the history of experimental approaches toward ideas about sensory coding, using the relatively simple auditory system of acoustic insects. I will discuss the empirical evidence in support of Barlow's "efficient coding hypothesis", which argues that the coding properties of neurons undergo specific adaptations that allow insects to detect biologically important acoustic stimuli. This hypothesis opposes the view that the sensory systems of receivers are biased as a result of their phylogeny, which finally determine whether a sound stimulus elicits a behavioural response. Acoustic signals are often transmitted over considerable distances in complex physical environments with high noise levels, resulting in degradation of the temporal pattern of stimuli, unpredictable attenuation, reduced signal-to-noise levels, and degradation of cues used for sound localisation. Thus, a more naturalistic view of sensory coding must be taken, since the signals as broadcast by signallers are rarely equivalent to the effective stimuli encoded by the sensory system of receivers. The consequences of the environmental conditions for sensory coding are discussed.

Individual differences show that only some bats can cope with noise-induced masking and distraction

Anthropogenic noise is a widespread pollutant that has received considerable recent attention. While alarming effects on wildlife have been documented, we have limited understanding of the perceptual mechanisms of noise disturbance, which are required to understand potential mitigation measures. Likewise, individual differences in response to noise (especially via perceptual mechanisms) are likely widespread, but lacking in empirical data. Here we use the echolocating bat Phyllostomus discolor, a trained discrimination task, and experimental noise playback to explicitly test perceptual mechanisms of noise disturbance. We demonstrate high individual variability in response to noise treatments and evidence for multiple perceptual mechanisms. Additionally, we highlight that only some individuals were able to cope with noise, while others were not. We tested for changes in echolocation call duration, amplitude, and peak frequency as possible ways of coping with noise. Although all bats strongly increased call amplitude and showed additional minor changes in call duration and frequency, these changes could not explain the differences in coping and non-coping individuals. Our understanding of noise disturbance needs to become more mechanistic and individualistic as research knowledge is transformed into policy changes and conservation action.

Neurocognitive dynamics of near-threshold voice signal detection and affective voice evaluation

Communication and voice signal detection in noisy environments are universal tasks for many species. The fundamental problem of detecting voice signals in noise (VIN) is underinvestigated especially in its temporal dynamic properties. We investigated VIN as a dynamic signal-to-noise ratio (SNR) problem to determine the neurocognitive dynamics of subthreshold evidence accrual and near-threshold voice signal detection. Experiment 1 showed that dynamic VIN, including a varying SNR and subthreshold sensory evidence accrual, is superior to similar conditions with nondynamic SNRs or with acoustically matched sounds. Furthermore, voice signals with affective meaning have a detection advantage during VIN. Experiment 2 demonstrated that VIN is driven by an effective neural integration in an auditory cortical-limbic network at and beyond the near-threshold detection point, which is preceded by activity in subcortical auditory nuclei. This demonstrates the superior recognition advantage of communication signals in dynamic noise contexts, especially when carrying socio-affective meaning.

Singing in a silent spring: Birds respond to a half-century soundscape reversion during the COVID-19 shutdown

Actions taken to control the coronavirus disease 2019 (COVID-19) pandemic have conspicuously reduced motor vehicle traffic, potentially alleviating auditory pressures on animals that rely on sound for survival and reproduction. Here, by comparing soundscapes and songs across the San Francisco Bay Area before and during the recent statewide shutdown, we evaluated whether a common songbird responsively exploited newly emptied acoustic space. We show that noise levels in urban areas were substantially lower during the shutdown, characteristic of traffic in the mid-1950s. We also show that birds responded by producing higher performance songs at lower amplitudes, effectively maximizing communication distance and salience. These findings illustrate that behavioral traits can change rapidly in response to newly favorable conditions, indicating an inherent resilience to long-standing anthropogenic pressures such as noise pollution.

Innate chemical, but not visual, threat cues have been co-opted as unconditioned stimulus for social fear learning in zebrafish

Animals can use social information to detect threat in the environment. In particular, social learning allows animals to learn about dangers without incurring in the costs of trial-and-error learning. In zebrafish, both chemical and visual social cues elicit an innate alarm response, which consists of erratic movement followed by freezing behavior. Injured zebrafish release an alarm substance from their skin that elicits the alarm response. Similarly, the sight of conspecifics displaying the alarm response can also elicit the expression of this response in observers. In this study, we investigated if these social cues of danger can also be used by zebrafish as unconditioned stimulus (US) in learning. We found that only the chemical cue was effective in the social fear conditioning. We suggest that this differential efficacy of social cues results from the fact that the alarm cue is a more reliable indicator of threat, than the sight of an alarmed conspecific. Therefore, although multiple social cues may elicit innate responses not all have been evolutionarily co-opted to act as US in associative learning. Furthermore, the use of the expression of the immediate early genes as markers of neuronal activity showed that chemical social fear conditioning is paralleled by a differential activation of the olfactory bulbs and by a different pattern of functional connectivity across brain regions involved in olfactory processing.

Decision making in the face of a deadly predator: high-amplitude behavioural thresholds can be adaptive for rainforest crickets under high background noise levels

Many insect families have evolved ears that are adapted to detect ultrasonic calls of bats. The acoustic sensory cues indicating the presence of a bat are then used to initiate bat avoidance behaviours. Background noise, in particular at ultrasonic frequencies, complicates these decisions, since a response to the background may result in costly false alarms. Here, we quantify bat avoidance responses of small rainforest crickets (Gryllidae, Trigoniinae), which live under conditions of high levels of ultrasonic background noise. Their bat avoidance behaviour exhibits markedly higher thresholds than most other studied eared insects. Their responses do not qualitatively differ at suprathreshold amplitudes up to sound pressure levels of 105 dB. Moreover, they also exhibit evasive responses to single, high-frequency events and do not require the repetitive sequence of ultrasonic calls typical for the search phase of bat echolocation calls. Analysis of bat and katydid sound amplitudes and peak frequencies in the crickets' rainforest habitat revealed that the cricket's behavioural threshold would successfully reject the katydid background noise. Using measurements of the crickets' echo target strength for bat predators, we calculated the detection distances for both predators and prey. Despite their high behavioural threshold, the cricket prey still has a significant detection advantage at frequencies between 20 and 40 kHz. The low-amplitude bat calls they ignore are no predation threat because even much louder calls would be detected before the bat would hear the cricket echo. This leaves ample time for evasive actions. Thus, a simple decision criterion based on a high-amplitude behavioural threshold can be adaptive under the high background noise levels in nocturnal rainforests, in avoiding false alarms and only missing detection for bat calls too far away to pose a risk. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Signal detection and optimal acceptance thresholds in avian brood parasite-host systems: implications for egg rejection

Signal detection theory addresses the challenge of successfully identifying informative signals in noisy information contexts, allowing optimal behavioural decisions in diverse ecological contexts. The optimal acceptance threshold hypothesis proposed by Reeve (Reeve 1989 Am. Nat. 133, 407-435. (doi:10.1086/284926)) is an elegant theoretical model to predict the flexibility of acceptance thresholds for conspecific discrimination. This model has provided a robust framework used to explore recognition systems in a broad range of contexts such as animal communication, nest-mate discrimination or anti-parasitic host responses. In this review, we discuss key concepts related to the optimal acceptance threshold hypothesis applied to egg rejection decisions in avian brood parasite-host interactions. We explore those factors determining signal detectability in parasitized nests and how hosts adjust their rejection decisions to both the risk of parasitism and the potential costs associated with egg rejection. In addition, we discuss recent results that challenge some traditional assumptions of the optimal acceptance threshold hypothesis and provide a novel perspective to explore rejection decisions, such as the existence of single-threshold decision rules or acceptance decisions. An integrative view combining current evidence with traditional theory is needed to further advance the comprehension of optimal acceptance thresholds. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Inconsistent sexual signaling degrades optimal mating decisions in animals

Like political stump speeches and product advertisements, animal signals are highly repetitive and function to persuade receivers to adopt behaviors benefiting the signaler. And like potential constituents and consumers, receivers assess signals to inform their behavioral decisions. However, inconsistency in sexual signals is widespread and potentially injects uncertainty into mating decisions. Here, we show that females fail to make optimal mating decisions based on assessments of signal quality due to inconsistency in signal production. Natural levels of inconsistency markedly reduced female preference expression for a nonarbitrary signal of male quality. Inconsistency reshaped preferences even more profoundly than the better-known impediment of ambient noise. To our knowledge, this is the first demonstration of how inconsistent messaging degrades optimal decision-making in animals, with implications for understanding signal evolution.

Vocal plasticity in a reptile

Sophisticated vocal communication systems of birds and mammals, including human speech, are characterized by a high degree of plasticity in which signals are individually adjusted in response to changes in the environment. Here, we present, to our knowledge, the first evidence for vocal plasticity in a reptile. Like birds and mammals, tokay geckos (Gekko gecko) increased the duration of brief call notes in the presence of broadcast noise compared to quiet conditions, a behaviour that facilitates signal detection by receivers. By contrast, they did not adjust the amplitudes of their call syllables in noise (the Lombard effect), which is in line with the hypothesis that the Lombard effect has evolved independently in birds and mammals. However, the geckos used a different strategy to increase signal-to-noise ratios: instead of increasing the amplitude of a given call type when exposed to noise, the subjects produced more high-amplitude syllable types from their repertoire. Our findings demonstrate that reptile vocalizations are much more flexible than previously thought, including elaborate vocal plasticity that is also important for the complex signalling systems of birds and mammals. We suggest that signal detection constraints are one of the major forces driving the evolution of animal communication systems across different taxa.

Bright birds are cautious: seasonally conspicuous plumage prompts risk avoidance by male superb fairy-wrens

Increased predation risk is considered a cost of having conspicuous colours, affecting the anti-predator behaviour of colourful animals. However, this is difficult to test, as individual factors often covary with colour and behaviour. We used alarm call playback and behavioural observations to assess whether individual birds adjust their response to risk according to their plumage colour. Male superb fairy-wrens (Malurus cyaneus) change from a dull brown to conspicuous blue plumage each year, allowing the behaviour of different coloured birds to be compared while controlling for within-individual effects. Because the timing of colour change varies among males, blue and brown birds can also be compared at the same time of year, controlling for seasonal effects on behaviour. While blue, fairy-wrens fled more often in response to alarm calls, and took longer to emerge from cover. Blue fairy-wrens also spent more time foraging in cover and being vigilant. Group members appeared to benefit from the presence of blue males, as they reduced their response to alarms, and allocated less time to sentinel behaviour when a blue male was close by. We suggest that fairy-wrens perceive themselves to be at a higher risk of predation while in conspicuous plumage and adjust their behaviour accordingly.

White-crowned sparrow males show immediate flexibility in song amplitude but not in song minimum frequency in response to changes in noise levels in the field

The soundscape acts as a selective agent on organisms that use acoustic signals to communicate. A number of studies document variation in structure, amplitude, or timing of signal production in correspondence with environmental noise levels thus supporting the hypothesis that organisms are changing their signaling behaviors to avoid masking. The time scale at which organisms respond is of particular interest. Signal structure may evolve across generations through processes such as cultural or genetic transmission. Individuals may also change their behavior during development (ontogenetic change) or in real time (i.e., immediate flexibility). These are not mutually exclusive mechanisms, and all must be investigated to understand how organisms respond to selection pressures from the soundscape. Previous work on white-crowned sparrows (Zonotrichia leucophrys) found that males holding territories in louder areas tend to sing higher frequency songs and that both noise levels and song frequency have increased over time (30 years) in urban areas. These previous findings suggest that songs are changing across generations; however, it is not known if this species also exhibits immediate flexibility. Here, we conducted an exploratory, observational study to ask whether males change the minimum frequency of their song in response to immediate changes in noise levels. We also ask whether males sing louder, as increased minimum frequency may be physiologically linked to producing sound at higher amplitudes, in response to immediate changes in environmental noise. We found that territorial males adjust song amplitude but not minimum frequency in response to changes in environmental noise levels. Our results suggest that males do not show immediate flexibility in song minimum frequency, although experimental manipulations are needed to test this hypothesis further. Our work highlights the need to investigate multiple mechanisms of adaptive response to soundscapes.

Social information use in threat perception: Social buffering, contagion and facilitation of alarm responses

Group living animals can use the behavior of others as cues for the presence of threat in the environment and adjust their behavior accordingly. Therefore, different social phenomena that modulate the response to threat, such as social buffering, social transmission (contagion), and facilitation of alarm responses can be seen as different manifestations of social information use in threat detection. Thus, social phenomena that are functionally antagonistic, such as social buffering and social transmission of fear, may rely on shared neurobehavioral mechanisms related to the use of social information in decision-making about the presence of threat in the environment. Here, we propose a unifying conceptual framework for the study of social information use in threat perception based on signal detection theory.

Calling at the highway: The spatiotemporal constraint of road noise on Pacific chorus frog communication

Loss of acoustic habitat due to anthropogenic noise is a key environmental stressor for vocal amphibian species, a taxonomic group that is experiencing global population declines. The Pacific chorus frog (Pseudacris regilla) is the most common vocal species of the Pacific Northwest and can occupy human‐dominated habitat types, including agricultural and urban wetlands. This species is exposed to anthropogenic noise, which can interfere with vocalizations during the breeding season. We hypothesized that Pacific chorus frogs would alter the spatial and temporal structure of their breeding vocalizations in response to road noise, a widespread anthropogenic stressor. We compared Pacific chorus frog call structure and ambient road noise levels along a gradient of road noise exposures in the Willamette Valley, Oregon, USA. We used both passive acoustic monitoring and directional recordings to determine source level (i.e., amplitude or volume), dominant frequency (i.e., pitch), call duration, and call rate of individual frogs and to quantify ambient road noise levels. Pacific chorus frogs were unable to change their vocalizations to compensate for road noise. A model of the active space and time (“spatiotemporal communication”) over which a Pacific chorus frog vocalization could be heard revealed that in high‐noise habitats, spatiotemporal communication was drastically reduced for an individual. This may have implications for the reproductive success of this species, which relies on specific call repertoires to portray relative fitness and attract mates. Using the acoustic call parameters defined by this study (frequency, source level, call rate, and call duration), we developed a simplified model of acoustic communication space–time for this species. This model can be used in combination with models that determine the insertion loss for various acoustic barriers to define the impact of anthropogenic noise on the radius of communication in threatened species. Additionally, this model can be applied to other vocal taxonomic groups provided the necessary acoustic parameters are determined, including the frequency parameters and perception thresholds. Reduction in acoustic habitat by anthropogenic noise may emerge as a compounding environmental stressor for an already sensitive taxonomic group.

Spatial release from masking in insects: contribution of peripheral directionality and central inhibition

The detection, identification and discrimination of sound signals in a large and noisy group of signalers are problems shared by many animals equipped with ears. While the signaling behavior of the sender may present several solutions, various properties of the sensory system in receivers may also reduce the amount of signal masking. We studied the effect of spatial release from masking, which refers to the fact that the spatial separation between the signaler and the masker can contribute to signal detection and discrimination. Except in a limited number of cases, the contribution of peripheral directionality or central nervous processing for spatial unmasking is not clear. We report the results of a study using a neurophysiological approach in two species of acoustic insects, whereby the activity of identified interneurons that either receive contralateral inhibitory input (crickets) or inhibit one another reciprocally in a bilateral pair (katydids) was examined. The analysis of the responses of a pair of omega neurons in katydids with reciprocal inhibition revealed that spatial separation of the masker from the signal facilitated signal detection by 19-20 dB with intact binaural hearing, but only by 2.5-7 dB in the monaural system, depending on the kind of analysis performed. The corresponding values for a behaviorally important interneuron of a field cricket (ascending neuron 1) were only 7.5 and 2.5 dB, respectively. We compare these values with those reported for hearing in vertebrates, and discuss the contribution of spatial release from masking to signal detection under real-world chorus conditions.